Renew compression screw

ABSTRACT

An external fixator implant screw with an intercalated head, of which one embodiment for lag screw mode has a spherical head, and a partial thread. The spherical head in countersunk bone exerts concentric wide contact on insertion at various angles to surface. Optional canalization of central rod allows guide wire technique and optional mobility of head allows variable shaft length between head and thread. Another embodiment for use in basic implant mode, in a single bone fragment, at right angles to bone surface, has a conical head with limited basal contact on bone, the head being integrated to a solid rod and is fully threaded from head to leading tip. In both embodiments, the load transmission is renewable from outside in case of loosening, without reopening any wound. The implant is made of biocompatible material.

Continuation-in-Part Application of U.S. Utility patent application Ser.No. 09/643,294, Filed Aug. 22, 2000 now abandoned.

CROSS-REFERENCE TO RELATED APPLICATIONS

“Not applicable”

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

“Not applicable”

REFERENCE TO A MICROFICHE APPENDIX

“Not applicable”

BACKGROUND OF THE INVENTION

This relates to the field of Orthopedics and Trauma, human orveterinary. It can be used for other biologicalfixations/immobilizations such as botanical or other forms of life andfor tissues other than bone. It can be used in any engineering ormechanical endeavour in which it serves to hold and/or compress togetherfragments or masses of material together, while taking part in anoutside construct at a distance from the fragments.

Bone is living tissue. Bone fragments and surfaces can unite bybiological activity over a length of time, given proper conditions tofavor it. During this biological process of healing, the fragments haveto be held together continuously by various means, to achieve a finallyacceptable result for restoring function to the part.

The biological process is favoured by the following measures.

-   -   1. Immobilization of the fragments or surfaces attempting union.    -   2. Compression of the surfaces to increase the rigidity of        immobilization, and also promoting the biological process of        direct union without excessive callus formation.    -   3. Relieving recurrent stress and injury to the soft tissues and        neuro-circulatory mechanisms by immobilization.    -   4. Immobilizing only the healing parts, while encouraging        movement and activity of un-injured parts.    -   This has been attempted by the following methods.    -   A. Continuous traction    -   B. External casts of Plaster of Paris, other casting materials        and bracing.    -   C. Internal fixation.    -   D. External fixation.    -   E. Combined methods of fixation.    -   A. Continuous Traction:

This can restore the length of the limb, and further measures cancorrect rotation and angulation to an extent.

The following problems of this method seldom make it the preferredtreatment.

1. It is difficult to maintain the traction force continuously even withvery frequent attention.

2. Patient cooperation is difficult to achieve.

3. Due to intermittent loss of traction force, malunion may occur.Distraction and movement of fragments may cause delay or failure ofunion.

4. Circulatory problems can occur in the distal limb.

5. Wounds in the traction surface will not allow such treatment.

-   -   B. External Casts of Plaster of Paris, Other Casting Materials        and Braces.

The following problems are associated with them.

1. The immobilization is not rigid enough, when this is criticallyessential.

2. Encircling of the part causes sweating and discomfort in hotclimates.

3. Pressure sores can occur at pressure points, or due to insertion ofhard objects by patient for scratching. Bugs can get in.

-   -   5. Swelling of part within the cast can cause tightness and loss        of circulation or nerve function.        5. Loosening of cast occurs due to loss of swelling of the part,        or due to reduction of the thickness of the padding by moisture,        resulting in loss of reduction.    -   6. There is no access to any wounds inside, which may need        regular attention, except by cutting out windows or leaving the        cast incomplete, which may jeopardize the immobilization, and        fracture position.        7. Uninvolved parts also are immobilized, a setback to recovery.

Due to these factors it can suffice only when rigid immobilization isnot critically important, and usually in the absence of complicatingfactors of wounds and circulation.

-   -   C. Internal Fixation:

This may be applied along the side of a bone in the form of a plate andscrews of any preferred design. It allows accurate reduction when thisis most desirable; a bone graft can be added and lag screws driven asoften as feasible, for inter-fragmentary compression. Sliding devicescan be added to passively close any gaps arising later. Disadvantagesare as under:

-   -   a. Large exposures are required with relatively greater damage        to the soft tissues and bone circulation. Meticulous technique        may minimise this, yet the exposure is larger.    -   b. Compression between fragments once applied at operation wears        off within hours, depending on the quality of bone. There is no        possibility of renewing this compression once the wound is        closed over the device. It is not acceptable to re-anaesthetise        and re-expose the device repeatedly to re-tighten the screws.    -   c. Minimally invasive methods are performed through smaller        incisions but in order to place the plate directly on bone, the        periosteum and muscle have to be stripped blindly. The plate is        always unavoidably placed over some soft tissues, which melt        away under the pressure and loosen the plate. Loss of torque of        screws is unfavourable to biology of bone healing.    -   d. Plates are seldom favoured in compound fractures.    -   e. Fracture haematoma gets dispersed.

Internal fixation may be applied inside the medullary canal of bone inthe form of nails, pins and wires.

In closed nailing, the fracture haematoma is preserved.

The disadvantages are as under.

-   -   1. It is generally not applicable to children, due to growth        plates at the ends of bones.    -   2. It invades and occupies the bone from end to end, with the        possibility of spreading infection.    -   3. It is not stable to rotational forces, and interlocking        methods are not available for all situations.    -   4. In open nailing, the fracture haematoma is dispersed.    -   D. External Fixation:

This is most suited for open injuries of bone. The commonly used basicbone implant for the external fixator is the Schanz screw which can beinserted at a safe distance from the open wounds and fracture ends.

-   -   1. Access to wounds for frequent attention is easy.    -   2. There is no aggravation of injury to bone or soft tissue.    -   3. Safe corridor entries of screws prevent injury to        neuro-vascular structures.    -   4. In transverse fracture patterns, some compression can be        applied along the axis of the bone by dynamizing the construct.

The following limitations remain:

-   -   1. The basic implant e.g. the Schanz screw has a tendency to        loosen in bone, leading to instability and a proneness to        infection. Radial preloading of the implant in bone improves the        stability, by the technique of inserting a larger diameter screw        in a suitably smaller diameter drill hole, but the rod/bone        interface remains small.    -   2. The preload is only in one mode, viz. Radial, over a small        area.    -   3. After loosening, there is no way of regaining any degree of        stability in the same position, before the onset of infection.        If the loose screw had been initially placed in a mechanically        ideal site, then any next site for repositioning will be less        than ideal.    -   4. There is no lag screw effect of a Schanz screw, to exert        inter fragmentary compression. Inter-fragmentary compression        greatly enhances the stability, as well as the biological        process of union. Fragments can at most be splinted across, but        not drawn together and compressed in the lag screw mode, by the        conventional Schanz screw.    -   5. Taylor et al had patented a lag screw external fixator        implant with intercalated heads of various shapes, meant for        compression of fragments. All the heads patented by them had        engagement surfaces which were flat, their plane being at        90-degrees to the rod axis. Unless such a device is always        driven at 90-degrees to the bone surface, the head will stand up        on edge, with too much stress concentration at a point, leading        to crumbling of bone and loosening.

Also, two fracture surfaces are best compressed by a lag screw driven at90-degrees to the fracture plane and not at 90-degrees to the outersurface.

Since fracture planes run across a bone, they are at various angles tothe outer surface of bone and not parallel to it.

In actual use, the devices of Taylor et al would not be at 90-degrees tothe outer bone surface, being required to be at 90-degrees to thefracture plane. This would make the engagement surface of head stand upat an angle to the surface leading to mechanical failure and loosening.This may be the reason, why the device did not gain wide acceptanceamongst those skilled in the art, and for the paucity of any reports onits use in English literature.

If compression were applied at 90-degrees to outer bone surface, but notat 90-degrees to fracture plane, it will not only be inefficient but mayactually make the fracture surfaces to slide upon each-other rather thanbe compressed.

Taylor et al suggested that their device might also be used as“traditional external fixator” screw, in which case the head may beallowed to stand away from the bone surface, without engaging or loadingthe bone surface.

For such use as a “traditional” device, it must be considered that anintercalated head adds to the cost of the implant and needs a littlelarger entry incision. Unless the head is going to do some work, thesurgeon skilled in the art will not use it, preferring a conventionalimplant widely available, rather than pay for an intercalated head andnot put it to use. This could have ruled out its acceptance even as aconventional implant.

Taylor et al thus did not provide for the need of inserting a lag screwdevice at different angles to the outer surface of bone, which is thecrux of the art of lag screw fixation.

Taylor et al do not mention a spherical shape in their claims or anypart of text, nor does their discoid head with rounded edges stand up toa close geometric scrutiny for being “spherical on account of being asegment of a sphere”. In a similar way, none of the heads claimed byTaylor et al satisfy the criteria of being “conical”, when scrutinizedgeometrically. They describe a “Gemini capsule” shaped head at theirFIG. 6. This head they refer to as being “conical” on one occasion atcolumn 7, lines 32-34 of their application. This Gemini capsule head isin fact quite pear-shaped, and unlike any geometrical cone.

Ignoring Botanical interpretations of the term Cone, a Cone has beendefined at Merriam Webster Online Dictionary as:

“A solid generated by rotating a right triangle about one of itslegs—called also a right circular cone.” Or “A surface traced by amoving straight line passing through a fixed vertex.” Or, “A solidbounded by a circular or other closed plane base and the surface formedby line segments.”

“The “Gemini capsule” shape head seen at FIG. 6 of their patent has anarrower base which soon expands to a low belly in a curved line andthen tapers to the vertex.

Geometrical cones are widest at the base, tapering in straight lines tothe vertex in ever reducing cross-sectional areas, without any wideningfrom base upward. If curved lines are allowed in the geometry of a coneas in the Gemini capsule, with expanding and reducing cross-sections,then bizarre shapes can compete for the definition of a cone. Thepreferred conical head therefore as claimed in this application for abasic implant, is not anticipated by any device of Taylor et al, whoclaim only a lag screw device and not a basic implant.

-   -   E. Combined Methods of Fixation:

When any one method is inadequate to neutralize all the forces ofmuscular pull and gravity, another method is added onto the first. Forexample, in “mini-cum-external fixation” methods; one or two lag screwsused to hold together some fragments, are supplemented by an externalfixator construct, or by traction.

Even with such a supplementation, the lag screws can fail, because bythe blind stab-hole technique of insertion, there is always someinterposition of soft tissue between the screw head and the bonesurface. This soft tissue quickly undergoes pressure necrosis to loosenthe compression by loss of torque. The only residual control is theexternal fixator, which may not be adequate for the situation. Thecompression once lost cannot be regained.

SUMMARY OF THE INVENTION

The invention is aimed at preserving and augmenting the functions of theprimary bone implant of the external fixator in which, Axial and Surfacepreloads are added to the older method of Radial preloading of theimplant in bone. This has an added effect on the stability anddurability of the implant. The former two preloads are also renewable,because the screw can again be tightened after the first insertion. Thetriple preload widely distributes stresses away from the interface ofrod/drill hole, where loosening of a conventional implant occurs.

Another embodiment is a lag screw, which can be driven at a mechanicallysound right angle to the fracture plane, with renewable compression.Renewed and prolonged inter-fragmentary compression by external fixatoris a new advantage to the biology of bone healing.

This implant can also be used to supplement minimally invasive plateosteosynthesis with double advantage. The torque can be renewed to keepplate firmly on bone and the same implant can form an outside constructto augment the stability of an implanted plate.

All positive features of the prior implants are retained; permittingwound access, minimal incisions, safe corridor insertion, soft tissuepreservation, and no lengthwise invasion of medullary canal of bone. Nonovel disadvantages are introduced.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagrammatic front elevation of the invention with lag screwcapability, showing components.

FIG. 2 is a cross section of the prior art basic external fixationimplant in bone, showing Radial preload.

FIG. 3 is a front elevation of the preferred basic external fixationimplant in bone, showing the Surface preload and Axial preload inaddition to the conventional Radial preload, widely distributing stressover bone/implant interfaces.

FIG. 4 is the coronal plane view of a prior art two-piece internalfixation device commonly used to fix a proximal femoral fracture andhaving a screw capable of sliding within the barrel of an angled plate.

FIG. 5 is the coronal view of prior art external fixator holding thesame fracture as in FIG. 4. There is no compression of fracturesurfaces. Finer detail of construct is omitted.

FIG. 6 is the coronal plane view of the same fracture showing thepreferred lag screw device at right angles to the fracture plane forefficient compression. Both the lag screws are connected to the twopreferred basic implants below, through a construct of clamps and tubes.Finer detail of construct is omitted.

FIG. 7 is a multiple fracture of distal femur involving the knee joint.All fracture surfaces are compressed by preferred lag screws at rightangles across the fracture plane. An exception is the third from below,which is compressing three fragments across two fracture planes andcannot be at best angles to both fractures. Two basic implants areholding the proximal fragment, connecting the smaller distal fragmentsto the rest of the bone. Finer detail of construct is omitted.

FIG. 8 shows prior art device of Taylor et al driven at 90-degrees tofracture plane for best compression. This makes the head stand on edge,on the surface of bone.

FIG. 9 shows the preferred device driven at 90-degrees to fractureplane, with the Head 4 being a sphere, making wide concentric contact incountersunk bone surface.

FIG. 10 shows an undistorted enlargement of one prior art head of Tayloret al, for geometrical detail.

FIG. 11 shows how applying compression to fracture surfaces not at90-degrees to fracture plane may make the surfaces slide, instead ofbeing compressed.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is one embodiment of the lag screw implant. In the lag mode it ismainly in tension along its axis and other force vectors are neutralizedby the basic construct. It comprises:

1 being the tip at the first end, with guide wire 7 in the centralcanal. The tip shown is self tapping, but optionally a non-self tappingtip may be used.

2 is the short threaded section at the first end, the thread notextending to the head 4.

3 being the smooth screw shaft section meant for gliding through thedrill hole in the fragment nearer to head, allowing lag screwcompression.

4 is the spherical head for engaging a countersunk surface of thefragment nearer to head. The head may be integral with the rod or may bemobile for fixation at a desired level to rod 5.

Such refixation is provided by means of a transverse screw through thehead and drive shaft possessing holes 15, at intervals for screwpassage.

5 is the unthreaded drive shaft, which serves for driving the device inor out, also for being secured to an external fixator construct througha clamp; as well as for subsequent turning of device to renew thetorque, in case of loosening. It has transverse holes 15, at intervalsto accommodate a screw through a mobile head for refixation of head. Theholes may be spirally arranged, instead of linear as shown.

6 is the second end which is outside, with a means for gripping, shownas a milled surface in the figure. The grip can optionally be quickcoupling, or faceted by triangulation, to suit the gripping handle orchuck.

7 is the guide wire passing from any one end, through the entire rod andout at the other end. It helps to direct the device at the best angleacross the fracture plane.

FIG. 2 shows the cross-section 11 of a prior art Schanz screw driveninto a drill hole of a suitably smaller diameter, in bone B. Thisgenerates a Radial preload 8 at the implant/bone interface. Such apreload should not be excessive as judged by those skilled in the art,otherwise micro-fractures in the drill hole wall will occur, with quickloosening.

FIG. 3 shows the preferred basic implant inserted in bone B. It is forgripping a fragment, not for lag-screw mode.

1 is the non-self tapping tip at the first end. Pre-tapping the threadwith tap is known to prolong the durability of a screw againstloosening.

2 is the fully threaded section from the first end to the head,providing maximum interface with bone.

4 is the conical head with base towards the bone, with a blunt serratedbasal surface for making blunt limited contact on bone at 14. The headexerts a Surface preload 9 on bone surface and on tightening the screwthe implant is Axially tensioned in direction shown at 10, within thebone B. The vertex of the cone where the cone converges to a point, isincorporated in the shaft, as the conical head is fixed to the shaft andis continuous with it.

When it became known that bone plates in intimate contact with bonesurface interfere with the surface nutrition, limited contact plateswere designed with better results. The same principle had not beenapplied to screw heads. Now, to allow blood supply to reach thedrill-hole margin under the conical screw head, a limited contactsurface is incorporated where it engages the bone surface, in thepreferred device. Sharp serrations are avoided, to prevent too muchstress concentration on bone. Nutrition favours bone integrity.

The two preloads, Surface 9 and Axial 10, are in addition to Radialpreload 8 of prior art.

The head is integral to the rod for stability, disallowing anymicro-movement.

Basic implants are driven at right angles to bone surface, in a singlefragment, when the leading base of the head 4 makes all round evencontact on bone B.

The device is not canalised, for greater strength against lateralbending.

The overall dimensions of the device and its parts are made to suit thesize of bone, the size of fragments and the depth of bone from skin.Thus in a superficial bone like the tibia, the conical head will besquat, to contain it within the skin. In a deeper bone like the femurthe cone will taper taller, for easy removal by spreading the softtissues.

FIG. 4 shows a common variety of hip fracture with fracture plane F,giving rise to two fragments B and B′.

A frequently used two-piece device, a sliding hip screw X in barrel Y,is holding the fracture reduced and compressed by screw Z at the time ofoperation.

This compression wears off in time after the skin S is closed, with nopossibility of recompressing. The screw may fail to slide causing apersisting gap and non-union.

FIG. 5 is the same fracture F as in FIG. 4, held with prior art externalfixator comprising pins P, clamps 12 and tube 13. The Fragments B and B′are splinted over the upper two Schanz screws, but no compression can beachieved, because fragment B′ can slide out on the smooth pin section.The lower two Schanz screws are the basic implants, which can beinserted with Radial preload. When Radial preload tapers off, it cannotbe renewed unless screw is reinserted at a new site. No other preloadsare possible in this prior art design. Persisting gap at fracture sitemay lead to delay or failure of union.

FIG. 6 is the same fracture F between B and B′, stabilized with anexternal fixator using the preferred device. The upper two screws in lagmode help active compression at fracture site, which can be renewed byloosening one screw at a time at the clamp, turning it tighter, andretightening the clamp. The lower two screws are the basic implants forcompletion of the construct, driven with Radial preload. The head exertsSurface preload on the bone, adding to the lateral stability of theimplant. In addition, there is an Axial preload created along the lengthof the screw, tensioned on driving it tight. These preloads are same asshown in FIG. 3. Thus, there is a wider distribution of stressescompared to the prior art Schanz screw in which all stress is borne atthe rod/drill hole interface. The axial and surface preloads arerenewable at intervals, without any repeated anaesthesia or exposures.The preloads are also mutually protective. Clamps 12 connect to tube 13.

FIG. 7 shows a multiple fracture in the lower one third of the femur,involving the knee joint, reduced and held by a fixator using theimplants of invention. The fragments are B, B′, B″, B′″ and B″″. Thefracture planes are F, F′, F″, F′″, F″″; of which F″ involves the jointsurface.

The lower five implants are in lag mode, the partial thread engagingonly one fragment near the first end. The lag screws are at right anglesto the fracture planes, except the third from below. Since this iscompressing three fragments B″″, B′ and B″ across two fractures F″″ andF′, it is at best possible angle that the situation permits. The detailof the head and countersink are shown at FIG. 9.

The upper two implants are in basic mode to control B, the uppertwo-thirds of the bone. They are at right angles to the bone surface forbest mechanical advantage. All implants are connected through clamps 12to the tube 13.

All the implants have capability of renewable stability and compression,which gives quicker union. This protects against instability, biologicalfailure, and sepsis. The lowest two lag screws compress the articularfragments B″ and B′″ to each other and stabilize them to the rest of thebone. The screws need not all be in the same plane as shown in thefigure. They may tilt into different planes and require creativeinterconnection to other construct components.

FIG. 8 shows one prior art device of Taylor et al seated in bone B. Thefracture plane F runs at about 45-degrees to the outer surface of bone.For optimum compression, the force vector should be at 90-degrees tofracture plane. When their device is so driven, the head 4 gets tilted,being flat at the engagement surface. This leads to much localizedpressure on a small area and will lead to mechanical failure of bone bymicro fractures. Taylor et al have patented heads of many shapes, all ofwhich have the same flat engagement plane to engage the bone surface.

FIG. 9 shows the preferred lag screw device with a sphere-like head 4,driven at 90-degrees to fracture plane F. Being tilted at about45-degrees to the surface, it still has a wide concentric contact withthe countersink in bone. Wide distribution of compressive load ofsphere-like head reduces likelihood of mechanical failure of bone.

FIG. 10 shows an undistorted enlargement of the head of Taylor et al,which appears in FIG. 4 of their application. Enlargement has renderedmore noticeable the smaller irregularities of the original drawing butthe geometry is preserved.

The head, when examined for attributes of being spherical, may vie forthe category of “spherical, being a segment of a sphere enclosed by twoparallel planes”. The two parallel planes of the head being equal, theymust lie at equal distance on either side of an equatorial plane of theoriginal sphere, the segment in question being bisected lengthwise bythe diameter of the parent sphere/circle.

The diameter of the original sphere from which the segment is derived,would then be the diameter of the segment. If a circle O is describedwith its centre I and diameter D the same as that of such a spheresegment, then this circle should represent the original sphere fromwhich the segment is derived. The surface of the sphere and theperipheral curve of the segment should then exactly coincide. This isnot the case with the head under scrutiny. The curved periphery of thehead is part of a much smaller circle/sphere than the curvature of thecircle/sphere from which it should have been derived.

The head can then at best be described as “discoid, with roundedperiphery” rather than as “spherical, being segment of a sphere”. Thisprior art therefore does not qualify for anticipating the present claimof a device, with spherical head shaped like a sphere. Even if it hadqualified for the definition of being spherical, being a segment of asphere, different segments of a sphere will have such varying mechanicalattributes, that many other claims would still be admissible dependingon how the segments are derived and what are their engagement surfaces.Different segments of the same sphere can look and mechanically functionvery differently. It can only be a proper sphere like the preferredhead, which can present the same spherical surface all round, with thesame mechanics all round. The segments of that do not share allqualities of a sphere.

FIG. 12 shows compression applied to fracture surface of bone B not at90-degrees to fracture plane F, but at 90-degrees to the outer surfaceof bone. The fracture surfaces are sliding instead of being compressed.

1. A bone screw in lag mode, comprising a short thread at its leadingend for engaging only one fragment of a bone, and a smooth shaft beyondthe short thread to an integrated totally spherical head, whereby theshaft is slidable without catching, and wherein the head on engagementmakes a wide contact at a matching countersink on a bone surface, and atany angle with the bone surface thereby achieving compression at rightangles to a fracture plane; and an extended smooth shaft beyond the headhaving a grip at an outer end to apply a known device for turning thebone screw in or out; and being connectable to an external fixationconstruct by a clamp, and wherein the outer end enables the gentleremoval of the bone screw by spreading soft tissue, and wherein the bonescrew is made of a bio-compatible material.
 2. The bone screw of claim1, and further comprising a canal throughout its length, whereby a guidewire may pass from one end of the bone screw to the other end.
 3. Thebone screw of claim 1, and further comprising a mobile spherical headfixed by means of a transverse screw through the head and one of aplurality of a holes in the outer end of the shaft.
 4. The bone screw ofclaim 2, and further comprising a mobile head fixed by a transversescrew through the spherical head and at least one of a plurality ofholes in the outer end of the shaft.